1. Field of the Invention
The present invention pertains to the field of olefin polymerization in slurry reactors. More particularly, the present invention pertains to a process for transferring a polyolefin/hydrocarbon solvent slurry from a first, hydrogen rich slurry reactor to a second reactor operated with a low and carefully controlled hydrogen concentration with concomitant removal of hydrogen from the first reactor discharge slurry stream. Within the context of the present invention, hydrogen refers to diatomic hydrogen.
2. Background Art
Polyolefins, particularly polyethylene and polypropylene, are produced in large amounts worldwide by polymerization of olefin monomers. The polyolefin products are employed in numerous products produced by a variety of fabrication processes, including, inter alia, extrusion, injection molding, resin transfer molding, blow molding, roto molding, and the like. Both the ultimate product physico-chemical requirements as well as the various fabrication processes dictate that numerous types of polyolefin resins exist. Thus, homo-, co-, and terpolyolefins are common, in various molecular weight grades. In addition, these polyolefins are frequently compounded, not only with other polyolefins having different physicochemical properties, but also additives such as plasticizers, impact modifiers, antioxidants, flame retardants, UV stabilizers, dyes, pigments, fillers and the like.
Three major types of polyolefin polymerization are known: solution, slurry, and gas phase. Each type has numerous variants. Gas phase and slurry processes are well suited for preparation polyethylene polymers of a wide range of densities, and provide a high percentage of polyethylene currently produced. Slurry reactors, as described in greater detail hereafter, employ a hydrocarbon “solvent” as a slurry medium in which the polyolefin product is substantially insoluble. While solution processes may utilize soluble catalysts, gas phase and slurry reactors generally require supported catalysts where the catalyst is present, at times in the presence of supported cocatalysts or “activators,” on an inert support such as finely divided silica.
Conventional polyolefin polymerization generally produces polyolefins with a substantially monomodal molecular weight distribution, which may be narrow, intermediate, or broad. The molecular weight distribution for monomodal polymer populations is generally reported as the polydispersity, Mw/Mn, where Mw is the weight average molecular weight and Mn is the number average molecular weight. The polydispersity of polyolefin polymers ranges upwards from above the theoretical minimum of 2.0 for catalytic synthyses processes to as much as 100 or more.
Multimodal resins have more than one significant population of molecular weights. The molecular weight distribution can best be visualized by viewing a gel permeation chromatography plot of the resin. Multimodal populations of molecules will exhibit two or more rather well defined peaks. The terms “bimodal” and “multimodal” are well known to those skilled in the art. The term “multimodal” as used herein includes bimodal resins. Polyolefins with a multimodal molecular weight distribution, preferably an essentially bimodal molecular weight distribution, have great value for certain products and processes.
Multimodal resins may be prepared by physical blending two or more resins having different molecular weight distributions. One disadvantage of such blended resins is that blending constitutes an additional process step. Moreover, the blending must be performed in such a way that a homogenous product is obtained. The blending operation not only adds additional cost to the resin, but moreover, the multimodal resins produced by blending have generally inferior physicochemical properties as compared to multimodal resins having been produced by “in situ” routes.
Preparation of polymer blends in situ avoids physical blending and its disadvantages. Four types of in situ multimodal polymer production may be conceptualized. In a first process, a single reactor is employed with two distinctly different catalysts, each catalyst prepared separately on its respective support. One catalyst is selected to provide a higher molecular weight product than the other catalyst. In such a process, two distinctly different polymers are created, and the product is distinctly heterogenous. Such products are generally inferior in their processing properties, especially for applications such as film production.
In a second process, a single reactor is again used, but two different catalysts are contained on the same support, i.e., so-called “dual site” catalysts. As a result, two different polymers grow from the same catalyst particle. The resultant polymer may be described as “interstitially mixed.” A much greater degree of homogeneity in the polymer product is thus obtained at the expense of more complex catalyst preparation. Although this process offers advantages in capial and installed costs relative to multi-reactor processes, the design and synthesis of dual site catalysts is difficult. An additional process disadvantage is that use of a single reactor reduces the number of process parameters that can be manipulated to control polymer properties.
In a third process, cascaded reactors are employed, and additional catalyst is added to the second reactor. The polymer particles from the first reactor continue growth in the second reactor, although at a slower rate. However, new polymer growth begins on the newly added catalyst. Hence, as with the first process described, a heterogenous polymer product is obtained, with the same deficiencies as described previously for such products.
In a fourth process, cascaded reactors are again employed, but catalyst is added only to the first reactor. The supported catalyst associated with the first reactor polymer contain further active sites which initiate polymerization in the second reactor. The second reactor polymerization parameters are adjusted to establish a different polymerization rate and/or molecular weight range as compared to the first reactor. As a result, an interstitially mixed polymer is obtained.
EP-A-0057420 represents an example of a cascaded slurry process wherein catalyst is introduced only into the first reactor. However, molecular weight is regulated by the presence of hydrogen in both reactors, with the second reactor having higher hydrogen concentration than the first reactor, thus limiting the types of interstitially mixed polymers which may be produced. Polymerization at lower hydrogen pressure in the second reactor is not possible. In addition, the polymer formed in each reactor is limited to a specific weight percentage range relative to the weight of the final product.
U.S. Pat. No. 5,639,834 (WO 95/11930) and WO 95/10548 disclose use of cascaded slurry reactors in which the catalyst feed is also limited to the first reactor. In both references, the first reactor polymerization is conducted at very low hydrogen concentration, and all olefin comonomer is incorporated therein. The second polymerization is conducted at high hydrogen concentration with no comonomer feed. U.S. Pat. No. 5,639,834 additionally requires that the takeoff from the first reactor be by way of a settling leg. Continuous takeoff is said to produce inferior products. These processes do not allow operation of the second reactor at lower hydrogen concentration than the first reactor. Moreover, limiting olefin comonomer incorporation to only the first reactor limits the types of polymers which may be produced.
WO 98/58001 discloses that significant advantages in polymer properties are achievable by conducting a two-stage polymerization, the first stage at high hydrogen concentration and low comonomer concentration and the second stage at low hydrogen concentration and high comonomer incorporation. The reactor may be a single reactor or a cascaded reactor system, the latter being preferred. A single catalyst, introduced into the first reactor, may be used. Lower hydrogen concentration in the second stage is achieved by limiting the choice of catalysts to those which rapidly consume hydrogen. Cessation of hydrogen feed thus causes the hydrogen concentration to fall rapidly between stages. The inability to add significant comonomer to the second stage or to limit comonomer incorporation in the first stage detracts from the ability to produce a wide variety of polymers. Moreover, the catalyst choice is limited to those which consume hydrogen when a single catalyst is used.
U.S. Pat. Nos. 6,221,982 B1 and 6,291,601 B1 disclose cascaded slurry polymerizations where at least two distinct catalysts are employed. In U.S. Pat. No. 6,221,982, a Ziegler-Natta catalyst is employed in the first reactor with high hydrogen concentration and no or low comonomer incorporation. A hydrogen-consuming catalyst with low olefin polymerization efficiency is introduced downstream into the first reactor product stream. As a result, hydrogen is consumed prior to reaching the second reactor, wherein the polymerization is conducted at substantially zero hydrogen concentration. The second stage employs significant olefin comonomer. U.S. Pat. No. 6,291,601 is similar, but employs a metallocene catalyst in the first reactor.
Both the U.S. Pat. Nos. 6,221,982 and 6,291,601 processes as well as the process of WO 98/58001 are inefficient in both monomer usage and thermal loading, since the hydrogenation reaction consumes ethylene, producing ethane by hydrogenation. In addition to the increased thermal loading created by this reaction, the ethane produced is an inert gas which must be purged from the system. Moreover, in the U.S. Pat. Nos. 6,221,982 and 6,291,601 processes, an additional relatively expensive hydrogenation catalyst which contributes little to polymer production must be added. Finally, all three processes require substantially homopolymerization in at least the first reactor, thus limiting the types of polymers which may be produced.
It would be desirable to use series-configured slurry reactors wherein hydrogen is introduced into a first slurry reactor to produce a low molecular weight first polymer, following which this first polymer then introduced into a second reactor operated at lower hydrogen concentration, without the requirement of employing a catalyst which specifically encourages hydrogenation. The higher molecular weight polymer produced in the second reactor will be interstitially mixed with previously produced low molecular weight polymer particles which still contain active catalyst. In general, the mass flow of hydrogen contained in the slurry polymer entering the second reactor must be lower by a factor of at least 50, preferably at least 100, from the mass flow of fresh hydrogen to the second reactor, otherwise control of polymer product melt flow index becomes difficult. To effectuate such a process, therefore, hydrogen introduced in the first slurry reactor must be efficiently removed from the first reactor product stream, as the hydrogen concentration in the second reactor will be far lower. In the case of high boiling solvent slurry media such as hexane, removal of hydrogen is relatively straightforward, typically being accomplished with a single stage flash. However, in the case of low boiling solvents such as propane, butane, and isobutane, efficient separation of hydrogen is difficult.
U.S. Pat. No. 6,225,421 B1 discloses use of cascaded reactors wherein ethylene is homopolymerized in the presence of hydrogen in a first reactor, hydrogen is physically separated from the first reactor product stream, and the product is copolymerized with 1-hexene and additional ethylene at reduced hydrogen concentration in the second reactor. However, the patent contains no disclosure of any apparatus suitable for removing hydrogen from the first reactor product stream. Moreover, the necessity to restrict the first polymerization to homopolymerization is limiting.
It would be desirable to provide a cost-effective apparatus suitable for removing hydrogen from the product stream of a first reactor operating at higher hydrogen concentration than a second reactor in series with the first. Use of such an apparatus in a cascaded slurry polymerization process would enable employing a light solvent, hydrogen-mediated slurry reactor in series with a second slurry reactor to produce a multimodal polyolefin polymer without reacting away hydrogen through the use of hydrogen-consuming polymerization catalysts or separate hydrogenation catalysts. It would further be desirable to provide a hydrogen removal process which can accommodate comonomer incorporation in any reactor of the reactor battery.